The present invention relates to a carrier to be mixed with a toner to provide a two-component developer for electrophotography and a developer containing the carrier.
A two-component dry developer for electrophotography comprises a toner and a carrier. The carrier is mixed with the toner in a mixing zone of a developing machine to give a desired charge quantity to the toner and carries the charged toner onto an electrostatic latent image formed on a photoreceptor to form a toner image. The developer is replenished with a supplementary amount of a fresh toner for repeated use.
Applications of the developer of this type have been diversified with the wide spread of electrophotographic recording apparatus such as copiers, facsimiles and printers. In particular, market demands for higher image quality and a longer developer life have been increasing. With continued reduction in prices, a demand for size reduction of the recording apparatus is also growing.
To meet the demand for higher image quality, reduction of a toner particle size has been proposed. Since toner particle size reduction tends to be accompanied by reduction of chargeability, it is necessary for the carrier to have a reduced particle size to gain in specific surface area for imparting a sufficient charge quantity to the toner. However, a carrier with a smaller size has poorer flowability and is slower in triboelectrically charging a toner, resulting in a slower rise of charges, which can cause such problems as toner scattering.
Intensified mixing has been suggested to improve the flowability, which increases the stress on the developer. An increased stress will induce a so-called spent-toner phenomenon (adhesion of a toner to the surface of the carrier particles) and cause the resin coat to fall off the carrier core, thereby accelerating deterioration of the developer. As a result, the developer cannot maintain satisfactory developing performance for a long period of time.
Further, the recent tendency toward size reduction of recording apparatus has necessitated fulfillment of the above-mentioned demands for high image quality and a long life with a reduced amount of a developer. Size reduction of recording apparatus has also made it difficult to arrange sufficient members such as flow regulators in the mixing zone and the part of feeding a developer to a sleeve. Hence, it has been desired to make a developer which exhibits rapid triboelectric charging properties on mixing carrier and toner under such a stress that does not cause developer deterioration and which forms a uniform and soft magnetic brush on a sleeve.
Developers which have hitherto been proposed for high image quality, prevention of carrier adhesion to a photoreceptor (hereinafter simply referred to as carrier adhesion), and extension of a service life and appear relevant to the present invention are discussed below.
JP-A-59-104663 proposes a carrier having a saturation magnetization of 50 emu/g or less. However, merely decreasing saturation magnetization of a carrier leads to reduction of adhesion force to a magnet roller so that the carrier is not sufficiently prevented from being attached to a photoreceptor.
JP-B-4-3868 proposes a developer containing a so-called hard ferrite having a coercive force of 300 Gauss or more as a carrier. In using the developer, it is preferred that a recording apparatus, especially the magnet roller or the sleeve thereof, be designed specially for the hard ferrite having such a high coercive force. Therefore, the developer is not fit for size reduction of the apparatus. Further, the carrier has poor transportability due to its high coercive force and is not sufficiently mixed with toner. As a result, rapid triboelectrification is not secured, allowing toner to scatter.
Japanese Patent 3,005,120 teaches that the above problems are solved by controlling the magnetization intensity and the rise of magnetization in a magnetic field of 1000 Oe but is still unsatisfactory in preventing carrier adhesion. Additionally, because the chains of carrier beads with attached toner (magnetic brush) are short, they hardly make a bridge between magnetic poles on a sleeve and are liable to come into contact with a photoreceptor while standing. That is, the magnetic brush is not soft enough to achieve high image quality.
JP-A-6-332237 proposes controlling developer flowability constant so as to improve developer transport properties. The inventors specify the flowability per unit weight. However, since a developer in an actual developing zone (i.e., on a sleeve) flows in a given capacity, sufficient effects are not obtained by the proposed flowability control. Further, where the residual magnetization and the coercive force are too low, the chains of carrier beads easily become too sparse to exert sufficient developing ability, and it is difficult to control the developer flowability in the developing zone, which makes it hard to retain high image quality for an extended period of time.
JP-A-7-175264 discloses a developer free from carrier adhesion and an edge effect, in which a carrier core having a controlled flowability and a controlled apparent density is coated with a non-uniform resin coat. Use of the proposed carrier core, however, turned out to result in deteriorated carrier flowability in a mixing zone. The carrier and a toner are not mixed sufficiently for adequate triboelectrification, and fog and toner scattering can result. This drawback is conspicuous in a smaller-sized apparatus.
JP-A-7-175265 describes a developer comprising a toner prepared through a specific suspension polymerization process and a carrier with controlled magnetic characteristics, which was proposed to secure environmental stability of developing performance and to obtain fog-free clear images. While the proposed carrier shows some effect on carrier adhesion, the degree of accomplishment is not enough to fulfill the recent market demands for alleviation of a spent-toner phenomenon and for high image quality. In addition, fog and toner scattering are not sufficiently prevented probably due to inadequate flowability in the mixing zone.
An object of the present invention is to provide a carrier for electrophotographic developer and a developer containing the carrier which exhibit satisfactory flowability in a mixing zone and on a sleeve, are free from carrier adhesion, toner scattering and fog, and provide high image quality in terms of image density, fine line or dot reproducibility, and the like.
The present inventors have extensively studied aiming at alleviation of carrier adhesion and toner scattering and improvement in image quality. As a result, they have found that the above object of the invention is accomplished by a developer with specific flowability.
The disadvantages accompanied with conceivable manipulations for obtaining high image quality are reviewed. The attempt of reducing the particle size of carrier and toner results in poor developer flowability and transport so that additional fresh toner is not rapidly triboelectrified by carrier, failing to produce a proper charge quantity. The insufficiently charged toner is transported to the sleeve as it is, which can cause fog or toner scattering. Intensified mixing for improving the rise of charging increases the stress on the developer, which can cause fall-off of the resin coat from coated carrier particles or a spent-toner phenomenon. Variations in charge quantity and developer resistance follow, leading to reduction of the developer service life. If an increased amount of a fluidizing agent is added to toner to improve the developer flowability thereby to accelerate the rise of charging, the fluidizing agent itself adheres to the carrier surface, which also leads to deterioration of the developer. Should the flowability be increased too much, localization of the developer will occur, failing to form a uniform magnetic brush on a sleeve. For example, the image density may differ between the upstream end and the downstream end.
It is an effective manipulation to decrease the magnetization of carrier so as to form a soft magnetic brush for improving image quality. Nevertheless, this means alone is not only incapable of obtaining sufficient image quality but can cause carrier adhesion. With a reduced magnetization, it is difficult for chains of carrier beads to make a bridge between magnetic poles on a sleeve and to form a soft magnetic brush. Being sparse, the carrier chains are incapable of supplying sufficient toner, resulting in a reduced image density. Increasing the coercive force of carrier to solve these problems is effective to obtain a sufficient image density, making the carrier chains denser and letting the carrier chains form arches over magnetic poles on a sleeve. However, too strong a coercive force rather results in formation of hard chains, causing image quality deterioration. It can also impair developer flowability on the sleeve, making toner supply poor, resulting in a failure of providing a sufficient image density. Further, an increased coercive force tends to continue influencing even after separation from the sleeve such that carrier particles are not easily disintegrated and mixed up with toner in a mixing zone. This will cause toner scattering or fog as stated above.
This being the case, it is important to control flowability of a developer in both a mixing zone having no magnetic field and a developing zone (on a sleeve) influenced by a magnetic field. Seeing that a developing unit has a definite capacity, the present inventors found it significant to properly control the flowability of a developer, especially a carrier, per unit volume.
Based on the above finding, the present invention provides a carrier for an electrophotographic developer which has a flowability index F1, represented by formula (1), of 63 to 75 sec/(50xc2x7cm3) and a flowability index F2, represented by formula (2), of 30 to 100 Oexc2x7g/cm3:
F1=ADxc3x97FRxe2x80x83xe2x80x83(1)
F2=ADxc3x97Hcxe2x80x83xe2x80x83(2)
wherein AD is an apparent density (g/cm3); FR is a flow rate (sec/50 g); and Hc is a coercive force (Oe).
The present invention also provides an electrophotographic developer comprising the above-described carrier and a toner.
Having appropriately controlled powder flow behavior which is specified in terms of flowability per unit volume in, and out of, a magnetic field, the carrier and the developer of the present invention exhibit satisfactory flow and transportability both in a mixing zone (with toner) and on a sleeve, achieve rapid triboelectrification with a sharp rise in the mixing zone, and form a uniform, soft and dense magnetic brush on the sleeve. Therefore, they are capable of providing high quality images free from fog without being accompanied with toner scattering and carrier adhesion for a prolonged period of time.